Update faq.adoc
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Adin Ackerman
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@ -4,6 +4,87 @@ These are a list of unsorted, commonly asked questions and answers.
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Please feel free to add items to link:https://github.com/embassy-rs/embassy/edit/main/docs/modules/ROOT/pages/faq.adoc[this page], especially if someone in the chat answered a question for you!
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== How do I even start?
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There are many ways to configure embassy and it's components for your exact application. The link:https://github.com/embassy-rs/embassy/tree/main/examples[examples] directory for each chipset demonstrate how your project structure should look. Let's break it down:
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The toplevel file structure of your project should look like this:
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[source,plain]
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----
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{} = Maybe
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my-project
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|- .cargo
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| |- config.toml
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|- src
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| |- main.rs
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|- build.rs
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|- Cargo.toml
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|- {memory.x}
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|- rust-toolchain.toml
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----
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=== .cargo/config.toml
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This directory/file describes what platform you're on, and configures link:https://github.com/probe-rs/probe-rs[probe-rs] to deploy to your device.
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Here is a minimal example:
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[source,toml]
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----
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[target.thumbv6m-none-eabi] # <-change for your platform
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runner = 'probe-rs run --chip STM32F031K6Tx' # <- change for your chip
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[build]
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target = "thumbv6m-none-eabi" # <-change for your platform
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[env]
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DEFMT_LOG = "trace" # <- can change to info, warn, or error
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----
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=== build.rs
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This is the build script for your project. It links defmt (what is defmt?) and the `memory.x` file if need be. This file is pretty specific for each chipset, just copy and paste from the corresponding link:https://github.com/embassy-rs/embassy/tree/main/examples[example].
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=== Cargo.toml
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This is your manifest file, where you can configure all of the embassy components to use the features you need.
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TODO: someone should exhaustively describe every feature for every component!
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=== memory.x
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This file outlines the flash/ram usage of your program. It is especially useful when using link:https://github.com/embassy-rs/nrf-softdevice[nrf-softdevice] on an nRF5x.
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Here is an example for using S140 with an nRF52840:
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[source,x]
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----
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MEMORY
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{
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/* NOTE 1 K = 1 KiBi = 1024 bytes */
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/* These values correspond to the NRF52840 with Softdevices S140 7.0.1 */
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FLASH : ORIGIN = 0x00027000, LENGTH = 868K
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RAM : ORIGIN = 0x20020000, LENGTH = 128K
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}
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----
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=== rust-toolchain.toml
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This file configures the rust version and configuration to use.
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A minimal example:
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[source,toml]
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----
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[toolchain]
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channel = "nightly-2023-08-19" # <- as of writing, this is the exact rust version embassy uses
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components = [ "rust-src", "rustfmt" ] # <- optionally add "llvm-tools-preview" for some extra features like "cargo size"
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targets = [
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"thumbv6m-none-eabi" # <-change for your platform
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]
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----
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== How to deploy to RP2040 without a debugging probe.
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Install link:https://github.com/JoNil/elf2uf2-rs[elf2uf2-rs] for converting the generated elf binary into a uf2 file.
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@ -36,3 +117,47 @@ For Cortex-M targets, consider making sure that ALL of the following features ar
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* `nightly`
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For Xtensa ESP32, consider using the executors and `#[main]` macro provided by your appropriate link:https://crates.io/crates/esp-hal-common[HAL crate].
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== Why is my binary so big?
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The first step to managing your binary size is to set up your link:https://doc.rust-lang.org/cargo/reference/profiles.html[profiles].
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[source,toml]
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----
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[profile.release]
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debug = false
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lto = true
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opt-level = "s"
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incremental = true
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----
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All of these flags are elaborated on in the Rust Book page linked above.
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=== My binary is still big... filled with `std::fmt` stuff!
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This means your code is sufficiently complex that `panic!` invocation's formatting requirements could not be optimized out, despite your usage of `panic-halt` or `panic-reset`.
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You can remedy this by adding the following to your `.cargo/config.toml`:
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[source,toml]
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----
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[unstable]
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build-std = ["core"]
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build-std-features = ["panic_immediate_abort"]
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----
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This replaces all panics with a `UDF` (undefined) instruction.
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Depending on your chipset, this will exhibit different behavior.
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Refer to the spec for your chipset, but for `thumbv6m`, it results in a hardfault. Which can be configured like so:
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[source,rust]
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----
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#[exception]
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unsafe fn HardFault(_frame: &ExceptionFrame) -> ! {
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SCB::sys_reset() // <- you could do something other than reset
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}
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----
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Refer to cortex-m's link:https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.exception.html[exception handling] for more info.
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